1. Field of the Invention
The present invention relates to a system for immobilizing a patient's skull during a medical diagnostic or treatment procedure, and more particularly to a stereotactic fixation system which permits precise and reproducible positioning of the patient's skull for focal irradiation or like medical procedures.
2. Description of Related Art
Modern methods of imaging, such as computerized tomography and magnetic resonance imaging, enable radiation oncologist to precisely delineate volumes of diseased tissue and normal anatomical structures. The data from imaging scans makes it possible to tailor radiation doses to a particular, identifiable treatment volume.
Focal radiation treatment usually involves the use of external beams, such as, for example, photon or proton beams. Such treatment requires accurate positioning and immobilization of the patient because a small misalignment in a patient's position with respect to the external beam can result in missing the target and/or delivering the high dose of radiation to normal, non-targeted tissue. The distance from the high-dose region of the external beam to its lateral and distal edges is often but millimeters.
Radiation treatment and pretreatment diagnostic studies also require reproducible positioning of the patient. Radiotherapy typically involves several diagnostic, planning and treatment stages. At each stage subsequent to the initial, accurate repositioning of the patient is essential. Most patients also require multiple treatments extending over several weeks. Variation of the treatment position between different sessions will decrease the efficacy of the treatment.
In an effort to provide accurate and reproducible positioning of a patient, stereotactic location devices have been developed. These devices rely on the assumption that the skull and its contents are rigid. Each anatomic point within the skull can then be uniquely identified when one knows the three spacial coordinates of that anatomic point.
Original neurosurgical and radiosurgical stereotactic location devices typically attach to the patient's skull by three or four pins surgically affixed. These invasive devices cannot be removed between diagnostic studies and treatment procedures, which therefore have to be performed within one day.
More recently, a relocatable stereotactic location device has been developed which employs a halo-ring which is invasively attached to the patient's skull and remains in place for several weeks unless the treatment is finished. An example of these devices is described in Clark, B. G., et al., "A Halo-Ring Technique for Fractionated Stereotactic Radiotherapy," The British Journal of Radiation, pp. 522-527 (June 1993). Such devices, however, are still invasive and may cause discomfort to the patient.
Another prior relocatable stereotactic device is described in Delannes, M., et al., "The Laitinen Stereoadaptor," Neurochirurgie, 1990, 36:167-175. This device can be quickly positioned on the patient's head using two ear plugs and a nasal support to locate the device on the skull. However, because these fixation points contact relatively soft tissue, such device inherently lacks the rigidity and reproducibility of bony or dental fixation.
Another prior stereotactic device has also used the upper teeth or alveolar ridge to position the stereotactic device on the patient's skull. The rigid connection between the upper teeth and the skull make the upper teeth a convenient and non-invasive reference point of the skull. This stereotactic device is disclosed in U.K. Patent Publication 2 213 066. Straps or similar structure are used in these devices to secure a mouthpiece of the stereotactic device within the patient's mouth. These straps, however, are awkward and time consuming to use, and can contribute to misalignment in head position. In addition, such straps cannot be easily and immediately removed in exigent circumstances. For instance, in cases where the patient chokes, vomits, or otherwise has trouble breathing, the mouthpiece cannot be quickly removed either by the health care provider or by the patient. In addition, a health care provider also may not immediately recognize the patient's condition because the stereotactic location device has immobilized the patient's head, and the patient may be unable to alert the health care provider of his or her condition.
Another disadvantage of using straps is that they may be variably located in the radiation treatment field, thus compromising the quality and reproducibility of dose delivery, especially where charged particles, such as protons, are used.
As noted above, the diagnostic, planning and treatment stages typically occur over the course of several secessions and involve the use of a variety of different imaging and radiation equipment. In collaborative programs between different institutions, the initial planning procedures typically occur at a facility different from that where the treatment procedures take place. These programs will become increasingly important because only a limited number of hospitals in the United States perform focal radiation therapy.
The orientation of the stereotactic fixation device relative to the reference axes of the scanning planes of the various diagnostic and treatment equipment used throughout the patient's course usually differs. Thus, even though the same fixation apparatus may be attached to the patient's skull during both the diagnostic and treatment stages, the patient's position with respect to the reference axes of the treatment equipment is not exactly replicated as planned.